U.S. Pat. No. 3,678,487--issued July 18, 1972 for a "Multi-Zone Incipient or Actual Fire and/or Dangerous Gas Detection System"--F. A. Ludewig and F. W. Van Luik--inventors, assigned to Environment/One Corporation of Schenectady, N.Y. (the assignee of the subject invention), describes and claims a multi-zone detecting system for incipient or actual fires and/or dangerous accumulations of potentially explosive gases. This known and proven incipient fire detector system is capable of monitoring the gaseous atmospheres of a number of different volumetric spaces (identified as zones) with a novel, sample-on-the-fly air sampling system that employs a selector valve assembly and sample gas conduit sub-system for continuously and sequentially supplying samples of the gaseous atmospheres from each of the zones being monitored to a centrally located particle detector of the Wilson cloud chamber type.